The invention relates to a process for drying melamine wet cake. In particular, the invention relates to a process for drying melamine wet cake which contains predominantly melamine crystals that are obtained from a concentrated melamine solution in a crystallizer. The invention particularly relates to a process for drying melamine wet cake containing less than 30 wt. % fines, the term fines being used herein to refer to particles having a diameter of less than 21 xcexcm.
A variety of processes for the production of melamine are described in Nitrogen, No. 139, September/October 1982, pp. 31-39. A number of the disclosed processes produce, at an intermediate step, a slurry of melamine crystals and water. These slurries are then subjected to a variety of additional processing steps necessary to produce a dry melamine product.
One of the melamine processes described in Nitrogen was the Stamicarbon process. In the Stamicarbon process, a caustic solution is added to this slurry and the resulting mixture is heated to dissolve the melamine crystals to obtain a concentrate melamine solution. The melamine solution is then filtered to remove insoluble materials, such as catalyst particles, to obtain a purified melamine solution.
This purified melamine solution is then fed into a crystallizer operating at reduced temperature and pressure to crystallize the melamine. The crystallized melamine and the remaining solution form a slurry that is concentrated further using a cyclone. The concentrated slurry is then fed into a centrifuge where the melamine crystals are separated from most of the remaining solution. The resulting product is composes predominantly of wet melamine crystals that is commonly referred to as xe2x80x9cwet cakexe2x80x9d by persons skilled in the art.
Following the Stamicarbon process, the wet cake is then passed upwards through a vertical pipe using a stream of hot air to produce a dried melamine powder. This type of dryer will be referred to herein as a xe2x80x9cflash dryer.xe2x80x9d below. The dried melamine powder and the hot air exiting the top of the flash dryer are then fed into a cyclone to separate the melamine powder. In addition to the Stamicarbon process described above, the Nitrogen article describes other processes in by which melamine wet cake may be obtained from the melamine slurry and subsequently dried.
The utility of the final dried melamine product is, however, related to more than simply the moisture content. Various bulk properties, in particular bulk density, compacted density, processing behaviour, and caking behaviour, are important factors reflecting a melamine product""s transportability and processability. An object of the present invention is to provide a process that results in improved bulk properties. This object was achieved by drying the wet cake in a contact dryer instead of in a flash dryer. As used herein, the term xe2x80x9ccontact dryerxe2x80x9d should be understood as referring to a dryer in which heat is applied to the wet cake predominantly via contact with the front surfaces, whether stationary and/or moving, of the contact dryer. The contact dryer surfaces are typically heated from the rear using a heating fluid such as steam or hot oil.
A number of types of drying apparatus, including tube bundle dryers, drum dryers, and tray dryers, can be configured as contact dryers in keeping with the present invention. A more detailed description of various contact dryers may be found in xe2x80x9cTrocknungstechnikxe2x80x9d, by K. Krxc3x6ll, Zweiter Band, Springer Verlag, 1978, pp. 364-73. Although a tube bundle dryer is preferred, the present invention is not so restricted and may be implemented with any suitable contact dryer.
According to the invention, the contact wall temperature in the contact dryer should be maintained at a temperature 100xc2x0 C. and 220xc2x0 C., preferably between 120xc2x0 C. and 160xc2x0 C. Drying processes utilizing wall temperatures below 100xc2x0 C. are less effective, uneconomical, and should generally be avoided. Similarly, drying processes utilizing wall temperatures greater than 220xc2x0 C. can produce unsatisfactory discolouration in the melamine.
Using a wall temperature of between 120xc2x0 C. and 220xc2x0 C., a residence time of between 0.1 and 10 hours, and preferably between 1 and 3 hours, is sufficient to dry melamine wet cake (moisture content of approximately 9 wt. % on a wet basis) to form a dry melamine powder (moisture content of less than 0.1 wt. % on a wet basis). As used herein, moisture content expressed in wt. % on a wet basis can expressed by the following formula using the weight of the remaining solvent (Ws) and the weight of the melamine (Wm):
[(Ws)/(Ws)+(Wm)]*100%
Dried melamine powder, for example, will typically have a moisture content of less than 0.1 wt. % on a wet basis. Although as described with reference to melamine wet cake having a moisture content of about 9 wt. % in which the solvent is water, it will be appreciated that the present invention is not so limited. It is contemplated that the present invention may be used effectively on a range of wet cake compositions, moisture contents, and solvents.
The applicants have also found that the present invention decreases the production of melamine fines during the contact drying process, particularly when compared with the conventional flash drying process. Without being bound to any exact theory, it is assumed that the improvement in the bulk density, compacted density, processing behaviour and caking behaviour achieved with the present invention is at least partly due to the reduced formation of fines during the drying process.
The applicants demonstrated this reduced formation of fines in a series of experiments comparing the flash drying and contact drying processes. As produced by the Stamicarbon process, melamine wet cake usually contains less than 6 wt. % fines, in particular less than 4 wt. % fines, and more particularly, less than 2 wt. % fines. The initial fines content was evaluated by drying melamine wet cake, obtained using the Stamicarbon process, on a laboratory scale for 16 hours at a temperature of 150xc2x0 C. As it may reasonably be assumed that no fines are formed during drying in the drying oven, it can be concluded that any increase in the number of fines results from the selected drying process.
When melamine wet cake produced via the Stamicarbon process having approximately 2 wt. % fines is dried with a flash dryer, the resulting dry melamine powder contains approximately 8 wt. % fines. This demonstrates that the number of fines increases dramatically during drying with a flash dryer.
Laser light diffraction, employing a Sympatec(copyright) Helos 12LA, was used for quantifying the percentage of fines present in the samples tested. The sample being tested was dispersed in air in a Rodos(copyright) dry feeder at a pressure of 20 mbar. In this case the melamine wet cake samples contained 9 wt. % moisture with the samples being dried to at least 0.1 wt. % moisture on a wet basis by the selected method. The reported moisture contents were determined by weighing a sample with Mettler(copyright) scales fitted with an LP16 heating element. After the initial weighing, the samples were heated and weighed again to determine the percentage of the original weight was lost due to evaporation of the solvent.
If, as the applicants suspect, the improvement in the bulk density, compacted density, processing behaviour, and caking behaviour possible through application of the present invention results from the reduced numbers of fines, the best results will be achieved with melamine wet cake having a sufficiently low initial level of fines. In those instances in which the initial level of fines is too high, restricting the formation of additional fines by using a contact dryer does not provide any great advantage. Without being bound to any scientific theory, the applicants believe that the manner in which the melamine crystals are formed effects the initial fines percentage in the wet cake.
The applicants have found that wet cake melamine having a low fines percentage can be obtained when the melamine crystals comprising the wet cake have predominantly and preferably crystallized from a melamine solution in a crystallizer. Drying such a wet cake with a contact dryer is particularly advantageous.
The melamine solution fed to the crystallizer may, however, contain some melamine crystals that were formed during earlier process steps. As a result, the final wet cake may contain a small portion of crystals that were not produced from the melamine solution in the crystallizer. It is preferred that the melamine crystals in a wet cake comprise at least 50 wt. %, and more preferably, 90 wt. %, melamine crystals formed from a melamine solution in a crystallizer. Examples of possible crystallizers are shown in xe2x80x9cPerry""s Chemical Engineers Handbookxe2x80x9d, by Don W. Green and James O. Maloney, 7th edition, Mc.Graw Hill, 1997, pages 18-44 to 18-55.
The Stamicarbon melamine production process described above is an example of a process that can produce a wet cake having a small amount of fines. It will be appreciated, however, that the invention is not restricted to drying a wet cake produced using the Stamicarbon process. It has been found that restricting the formation of fines by using a contact dryer instead of a flash dryer is advantageous when the percentage of fines in the wet cake is less than 30 wt. %, preferably less than 20 wt. %, more preferably less than 10 wt. %, and most preferably, less than 6 wt. %.